568 PROCEEDINGS OF SECTION J. 



In the Hooghly Bridge the caisson is somewhat similar in shape 

 to the Hawkesbury one, but ha,s a completely vertical outer skin, 

 and the three dredging spaces extend right across the sti'ucture, 

 occupying the semi-circular ends and the central portion. Weight 

 is obtained by concreting the two fifteen feet intermediate spaces 

 and by a brick lining around the semi-circular ends. In all these 

 cases the wells are of course filled up with concrete when the 

 bottom is reached. 



This review, as to what has been done, is now brought down to 

 the latest date, and a few remarks founded on practical experience, 

 of several years standing, of the majority of the systems mentioned, 

 may be made. 



No general principle can be laid down as to the preference -of 

 any one system over another. As in medicine, so in engineering, 

 and especially as regards foundations, not only does every disease 

 require its own physic, but even the same disease in different in- 

 dividuals demands separate intelligent treatment. 



Any remarks, therefore, that are made in this paper, must be 

 considered as thoroughly subordinate to this general principle, 

 that every case must be met and dealt with on its own merits. 



Having this in view, it must be first remarked that great 

 caution should be exercised in the use of screw piles. In railway 

 structures of any size they should certainly be avoided, except 

 under special circumstances favourable to their use. Not only 

 does the objection of difficulty of bracing below scourable beds, 

 already alluded to, arise, but the whole foundation is dependent 

 on the comparatively perishable material of the screw blade, 

 which in time may corrode unseen, and leave insufficient bearing 

 surface. Subsequent settlement of one pile may distort and 

 strain important parts of the upper structure. 



Failures already alluded to within the author's experience and 

 that of others have led to these opinions ; failures arising from 

 the system itself and not from any defects in the special designs 

 adojDted in these cases. 



With respect to cylindrical or well foundations, it has always 

 seemed to the writer strange that, with few exceptions, notably 

 that of the Tay Bridge, the system of brick wells should be confined 

 to India. There are plenty of rivers bridged elsewhere, in which 

 sandy beds, dry nearly all the year round, point unmistakably to 

 this expedient, in the Cape, for instance, where, notwithstanding 

 iron is preferred. Brick cylinders have the great advantage of 

 supplying their own weight, and of providing a more permanent 

 coating to the concrete core than metal does. Nor will it be 

 found, even when local skilled labour is expensive, that the former 

 costs more than the latter. 



